Here\'s how I understand about your entry into Canada .
Case 1 : For newly landed immigrants (less than five years) At POE these people have to show that they still have time to complete the obligation for staying a Permanent (or maybe that they have already complied with the obligation of physically staying in Canada for 2 yrs ) . You fall in this category , you did your landing in April 2001 and lets say that you left Canada immediately after that . You have time till April of 2004 to re-enter Canada because from April 2004 to April 2006 (when your five year period end from April 2001) you can comply with residency obligations of physically being present in Canada for 2 yrs in a 5 yr period .
Case 2 : For people who have been PRs for more than 5 years . At POE they have to show that within the past five years they were physically in Canada for 2 yrs .
For both the cases there are some exceptions where you can stay outside Canada for more time , for eg if you are working for a Canadian company outside Canada etc.